A. Summary of the Invention
The present invention provides a process and apparatus for generating a continuous one dimensional array of high strength, small diameter pitch based carbon fibers. In general, the present invention provides a blow spinning process which creates a blow spun product having melt spun product type characteristics at blow spinning speeds. Thus, the current invention improves product uniformity and production economics. While the present invention is particularly useful in blow spinning fibers from carbonaceous pitch, other uses will be apparent from the following disclosure.
B. Background of the Invention
Since blow spinning operations are well known in the art, they will be discussed only in general terms. A typical blow spinning process utilizes a spinnable substance, an attenuating media, normally a gas, and a spinning die containing fiber forming capillaries. During the spinning process, fibers exit the capillaries and are contacted by the attenuating media which draws or stretches each fiber increasing its length while decreasing its diameter. Since the attenuating media is normally a gas, the quench rate of the fibers is also affected.
Typically, fibers prepared by blow spinning processes are short discontinuous fibers having lengths of less than two inches. In contrast, melt spun fibers are generally continuous fibers with essentially infinite lengths. Normally, the fiber length of a melt spun fiber is chosen by design, not dictated by the process of producing the fibers. Additionally, melt spun fibers have better denier uniformity than blow spun fibers.
Several types of dies may be used for blow spinning fibers. In general, blow spinning dies are identified by the method of directing the attenuating gas into contact with the fibers. Two common designs are annular and slot dies. Specifically, in an annular die, the attenuating gas exits an annulus formed around each capillary and flows in a direction parallel to the fiber. Attenuation occurs due to the drag of the gas on the fiber. In slot dies, the attenuating gas exits slots on either side of a die tip contacting the fiber at an angle. The angle of contact, as determined by the die geometry, is optimized for the spinnable substance to obtain desired fiber characteristics. The present invention has equal application for all types of blow spinning dies.
Typical blow spinning methods allow the spun fibers to fall to a collection surface following attenuation. Depending upon the composition of the fibers, this method produces a two or three dimensional batt of short randomly arranged fibers. Fibers collected in this manner may contain undesirable bends and kinks resulting in lower fiber tensile strength. The primary cause of the bending and kinking of the fibers is believed to be the turbulence generated by the attenuating gas during the quenching of the fibers.
Those skilled in the art have recognized that the process of collecting fibers is a critical step in achieving high tensile strength fibers. Prior art methods for collecting blow spun fibers typically increased the number of bent and kinked fibers by collecting them in a three dimensional random batt. Recent U.S. Pat. No. 5,648,041 demonstrates the collection of fibers in a two dimensional batt. However, the means for achieving the two dimensional batt requires additional processing apparatus and strict control of the attenuating gas. As demonstrated by melt spinning processes, manufacturers have long recognized that unidirectional fiber lay-down or collection, for example on a spindle or bobbin, would produce the ultimate fiber properties. However, prior to the current invention, the means for achieving this goal in a blow spinning process had not been discovered.
The present invention overcomes the difficulties previously associated with blow spinning fibers by providing an apparatus and process for collecting continuous, blow spun fibers. Additionally, the process and apparatus of the present invention provides for unidirectional collection of the continuous fibers. As used herein, the term xe2x80x9ccontinuous fibersxe2x80x9d, means fibers having an essentially infinite length. Interruption in the length of continuous fibers generally occurs purposefully or due to a manufacturing problem. Further, fibers generated by the present invention have improved fiber uniformity and tensile strength. Finally, the present invention provides a blow spinning process which generates a melt spun type product at a rate equivalent to blow spinning.
The present invention provides an apparatus for blow spinning continuous, infinitely long fibers. The novel apparatus includes a blow spinning die having at least one capillary for forming a fiber and means for directing an attenuating gas into contact with the fiber as it exits the capillary. The apparatus also provides a means for placing tension on the fiber until it has thermoset or quenched, i.e. solidified. Further, the present invention optionally provides for the removal of gases and vapors from within the fiber array or bundle prior to collecting the fibers on a receiving device.
Further, the present invention provides a process for preparing continuous blow spun fibers. The inventive process prepares fibers by heating a spinnable substance and forming fibers by passing the substance through a spinning die. The fibers are attenuated by an attenuation gas. Following attenuation, the process precludes the formation of slack in the fibers by maintaining tension on the fibers by means of a tensioning device. The continuous blow spun fibers may be collected on the tensioning device or a subsequent device such as bobbin or windup.
The present invention additionally provides a process for preparing straight blow spun fibers from a carbonaceous pitch. The novel process utilizes a means for maintaining tension on the fiber to preclude slack prior to thermosetting or quenching the fiber. The present invention additionally includes the step of collecting said fibers as a warp sheet or traversed continuous filament tow on a receiving device. If necessary, the present invention provides for the removal of gases and vapors from the fibers prior to collecting the fibers on the receiving device. Preferably the gases and vapors will be removed by cross-flow ventilation applied to the fibers prior to the receiving device. Finally, the novel process will also be useful when spinning fibers from substances such as but not limited to, carbonaceous pitches, polyamides including nylons, polyesters such as xe2x80x9cDacron(copyright)xe2x80x9d, polypropylenes, polyurethanes including xe2x80x9cLycra(copyright)xe2x80x9d, polyaramides such as xe2x80x9cKevlar(copyright)xe2x80x9d and liquid crystalline materials such as mesophase pitch, solvated mesophase pitch and Zenite(copyright), where all trademarks are registered trademarks of E. I. du Pont de Nemours and Company.
Additionally, when spinning pitch based fibers, the present invention provides a continuous, straight, blow spun fiber. The fiber is free of kinks, bends and other collection flaws associated with blow spinning which lower the mechanical properties of the fiber. As a result, this invention yields a fiber with a significantly higher tensile strength.
Further, the present invention provides a traversed continuous fiber tow, a warp sheet and an unidirectional filament roving fabric prepared from continuous fibers made by a blow spinning process.